Jan. 4, 2012 -- More pregnant women are getting more than they may have expected, according to a report on 30-year trends in twin births.

In 2009, 1 in every 30 babies born in the United States was a twin, compared with 1 in every 53 babies in 1980, according to the “Data Brief” from the CDC's National Center for Health Statistics.

That represents a 76% jump in the twin birth rate.

Of note though, more than half of twins born from 1980 to 2009 were low birth weight, or less than 5 1/2 pounds, according to the report. One in 10 was very low birth weight, or less than 3 pounds, 5 ounces. Only 1 in 100 single babies is very low birth weight, notes researcher Joyce Martin, MPH.

When Martin first started studying twin birth rates years ago, she says, the impact on the mom's and babies' health tended to get overlooked. Instead, Martin says, “The idea was, ‘Isn't this cute?'” Most twins do well, she says, but they are more likely than single babies to experience complications related to prematurity and low birth weight, such as breathing problems.

Martin and her co-authors attribute the increase in the twin birth rate mainly to the expanding use of fertility treatments, namely ovulation stimulation medications and in-vitro fertilization.

The other major factor is the older age of mothers. In women who've conceived without the help of fertility treatments, the chance of having twins increases with age and peaks in the late 30s.

In 1980, about 20% of all births (singles and multiples) were to women aged 30 and older. In 2000-2009, they accounted for more than 35% of all births.

Twin Birth Rates Highest in Women 40 and Older

Because fertility treatments are more commonly used in older women, twin birth rates have been highest among women in their 40s since 1997, the authors write. In 2009, 7% of babies born to women 40 and older came in pairs, compared with 5% of births to women 35-39, and 2% of births to women under 25.

The tide may be turning, though. The pace of the increase in the overall twin birth rate, which on average rose 2% a year from 1980 until 2004, did slow to less than 1% annually from 2005-2009, the authors write.

“One contributing factor could be refinements in the use of fertility-enhancing technologies,” Martin says.

In addition, Martin says, the triplet birth rate has stabilized somewhat, although it is still much higher than it was before fertility treatments became available.

Fertility specialists have made a “concerted effort” over the last several years to minimize the chances of multiple births by reducing the number of embryos transferred in an in-vitro fertilization cycle, says Eric Widra, MD, a Washington, D.C., doctor who serves on the practice committee of the American Society for Reproductive Medicine.

In October, the practice committees of the ASRM and the Society for Assisted Reproductive Technology (SART) issued a report calling for greater use of “single embryo transfer” in in-vitro fertilization. Studies have shown that pregnancy rates with this approach are high in women who are good candidates. They include those under age 35, having more than one high-quality embryo available, who've previously gotten pregnant with in-vitro fertilization, who are in the first or second IVF treatment cycle, and who've received donor eggs.